Due to recent global warming, the polar icecaps have been melting, causing a rise in the sea level. Recent changes in climate have caused unusual weather phenomena around the world. Global warming is known to be attributed to increased greenhouse gas emissions. International agreements have been signed to restrict the emission of carbon dioxide. Attempts to suppress the emission of carbon dioxide by the introduction of carbon credits become economic issues in individual countries around the world. Efforts to reduce the emission of carbon dioxide have been directed towards the development of alternative energy sources (such as solar energy and wind energy) capable of replacing fossil fuels, and techniques for the capture and storage of carbon dioxide from fossil fuels while preventing the carbon dioxide from being released into the atmosphere. The latter techniques are called carbon capture and storage (CCS) techniques and are broadly divided into techniques for capturing carbon dioxide from power plants and steel mills and techniques for storing captured carbon dioxide in the soil or ocean.
The carbon dioxide capture techniques can be divided into post-combustion capture, pre-combustion capture, and oxy-fuel capture according to stages at which carbon dioxide is captured. The carbon dioxide capture techniques can also be divided into membrane separation, liquid phase separation, and solid phase separation techniques according to the principles of carbon dioxide capture. The membrane separation techniques use separation membranes to concentrate carbon dioxide, the liquid phase separation techniques use liquid absorbents such as amines or aqueous ammonia, and the solid phase separation techniques use solid phase absorbents such as alkali or alkaline earth metals.
The present invention is directed to a capture technique for continuously concentrating carbon dioxide contained in large amounts of flue gases from power plants and combustion furnaces by using an absorbent. The capture technique of the present invention belongs to post-combustion solid phase separation techniques for processing carbon dioxide contained in flue gases released after combustion.
The solid phase separation techniques are largely directed towards the development of solid phase absorbents having any absorbability for carbon dioxide and the capture process of carbon dioxide using solid phase absorbents. Carbon dioxide capture efficiency is greatly affected by the efficiency of absorption processes as well as the performance of solid phase absorbents.
Solid phase absorbents can be broadly classified into organic, inorganic, carbon-based, and organic-inorganic hybrid absorbents by the kind of their constituent materials. Solid phase absorbents can also be classified into physical absorbents and chemical absorbents based on their mechanism of carbon dioxide absorption. Representative examples of such solid phase absorbents include: amine polymer absorbents as organic absorbents; zeolite-based absorbents, alkali absorbents, and alkaline earth metal absorbents as inorganic absorbents; activated carbon absorbents modified with alkali metals as carbon-based absorbents; and MOF absorbents and porous silica absorbents grafted with organic materials having an amine group as organic-inorganic hybrid absorbents. Carbon dioxide is physically adsorbed to zeolite-based and carbon-based absorbents. Carbon dioxide is absorbed to the other absorbents through chemical reactions (Energy Environ. Sci. 2011, 4, 42. Chem Sus Chem 2009, 2, 796).
Such carbon dioxide capture processes using dry absorbents can be classified into pressure swing adsorption (PSA) processes and temperature swing adsorption (TSA) processes by the factors they use. The PSA processes use a pressure difference and the TSA processes use a temperature difference to desorb absorbed carbon dioxide. Generally, pressure swing adsorption processes using fixed bed sorption columns are advantageous in the capture of carbon dioxide on a small scale, and easy-to-scale-up temperature swing adsorption processes using fluidized bed sorption and desorption columns are advantageous in the capture of a large amount of carbon dioxide from power plants or large combustion furnaces.
The present invention is intended to capture a large amount of carbon dioxide in a continuous manner using a solid absorbent and is based on a temperature swing adsorption process using fluidized bed sorption columns and desorption columns.
Sorption columns and desorption columns used in temperature swing adsorption processes can be divided into bubbling fluidized bed columns and dilute fluidized bed columns according to the concentration of absorbents in operating stages. Absorbents are present at high concentrations in the bubbling fluidized bed columns and at low concentrations in the dilute fluidized bed columns. The application of such bubbling fluidized beds and dilute fluidized beds to sorption columns and desorption columns provides four possible combinations such as: i) dilute fluidized bed columns-dilute fluidized bed columns, ii) dilute fluidized bed columns-bubbling fluidized bed columns, iii) bubbling fluidized bed columns-dilute fluidized bed columns, and iv) bubbling fluidized bed columns-bubbling fluidized bed columns (“Fluidization Engineering”, D. Kunii and O. Levenspiel, Robert E. Krieger, 1977).
Korean Patent Publication Nos. 2005-0003767, 2010-0099929, and 2011-0054948 disclose fluidized bed processes for carbon dioxide capture that use dry solid absorbents based on the concept of temperature swing adsorption using dilute fluidized bed sorption columns and bubbling fluidized bed desorption columns. According to such solid phase separation processes based on the concept of temperature swing adsorption, however, a vast amount of energy of at least 2 GJ/t-CO2 is consumed to desorb carbon dioxide from absorbents. This energy consumption is a cause of increased capture cost, together with the cost of the absorbents. Thus, it is very important to develop a technology by which carbon dioxide can be effectively desorbed from absorbents with less energy, achieving reduced capture cost.